At last I have found a succinct an adequate description of the two opposite poles of the marriage debate. This is from a 1999 article by Robert George in First Things:

[On the traditional Judeo-Christian understanding, marriage is] a bodily, emotional, and spiritual union of one man and one woman, ordered to the generating, nurturing, and educating of children, marked by exclusivity and permanence, and consummated and actualized by acts that are reproductive in type, even if not, in every case, in fact. Marriage, for secularists, is a legal convention whose goal is to support a merely emotional union-which may or may not, depending upon the subjective preferences of the partners, be marked by commitments of exclusivity and permanence, which may or may not be open to children depending on whether partners want children, and in which sexual acts of any type mutually agreeable to the partners are perfectly acceptable.

There's a fair amount of arm-waving here, terms that need clarification, indeed an entire worldview that must be in place for each pole to make whatever sense it can. To his credit, a lot of what George writes in this article and elsewhere goes toward fleshing these things out. But let's just go with this rough account for now.

As I see it, the "marriage equality" movement understands itself as merely extending the scope of marriage without altering its essence. And if this is indeed what marriage is, it is hard to find a non-prejudicial reason to restrict it to heterosexual couples. To be sure, any pragmatic arguments that would point to the ill-effects of alternate unions will always be inconclusive for the purposes of law. They couldn't even prove that smoking causes cancer in a court of law, right?

But if George is right, the "marriage equality" movement subscribes to the secular notion of marriage he describes. It does not merely extend the essence of marriage but displaces it. And moreover, I would add, if the secular notion prevails, then there would be nothing about marriage that deserves the special status it enjoys in a secular state. It would be no more than a legal contract with no special distinction -- it could involve whomever and stipulate whatever. As such it would abolish any legal justification for marriage as a distinct institution. Marriage itself, one of society's greatest achievements, would effectually disappear.

The only way around this conclusion seems to be in either casting fatal doubt on some aspect the Judeo-Christian understanding of marriage (e.g., marriage, even back then, was never anything more than a mere construction). Or else in granting its lasting relevance, asking whether the set of defining traits of marriage attributed to it (bodily, emotional, spiritual union; one man and one woman, ordered toward procreation, etc.) necessarily cohere, or whether certain items can be broken off without affecting the whole. And that's where the heavy lifting comes in, including an adequate anthropology (one that does not regard the body as merely instrumental to the will) and a non-consumerist orientation toward marriage (there's a nice article on the this topic over at Ethika Politika). Indeed, issues that stand little chance in winning legal recognition, but are far too important to publicly ignore. As S. Adam Seagrave put it recently:

The problem, however, is that until Aristotelian-Thomists become politicians and judges or until politicians and judges become Aristotelian-Thomists (to adapt Plato’s phrase from the Republic), this sort of moral reasoning will find little purchase in the public arena, which is the sphere in which moral reasoning tends to have the highest stakes and the greatest influence on future generations.